A multiplexer is a device that selects from among multiple input signals received at its inputs and transmits the selected input signal on the multiplexer output. Multiplexers find utility, for example, in the context of serial data links, enabling multiple devices to alternately use the same serial data path. Conventional multiplexers typically place unselected inputs in a high impedance state. However, such an approach is not sufficient to isolate the multiplexer's output from its unselected inputs as data rates increase.
At high data rates (e.g., above several hundred MHz), parasitic capacitances in a multiplexer increasingly promote interference between signal paths, resulting in “crosstalk” between the signal paths of the unselected inputs and the signal path of the active input; with resulting interference at the multiplexer output. This may be understood with reference to the graph of FIG. 1 in which coupling from an unselected differential input of a multiplexer to the multiplexer's differential output is illustrated. As shown in the graph, the coupling or crosstalk becomes unacceptably high as the data rate approaches 10 GHz.
One approach to handling such crosstalk might be to insert additional circuitry in the input signal paths before they merge to increase the isolation between unselected multiplexer inputs. However, such an approach is likely to negatively affect signal integrity as well as latency; particularly for high data rate applications.